"To my knowledge, it is the oldest penis. I don't know of any older," Professor Siveter told the gathering

But penises have been around a lot longer than that, said Mr Martyn Robinson, a naturalist at the Australian Museum.

"It's just many don't fossilise well."

The fossilised penis belonged to an ostracod, an early crustacean related to crabs, shrimps and water fleas. Because ostracods and insects have an exoskeleton, body parts like the penis are more likely to fossilise.

Conversely, molluscs and worms are made of soft tissue.

"Although it's highly probable they had penises too, based on modern examples, finding any trace in the fossil record is unlikely," said Mr Robinson.

Ostracods have the longest sperm to body ratio of any animal known to man. An ostracod one millimetre long can produce a single sperm 10 millimetres long. And like most animals, they adapted to accommodate the specifics of their reproductive system. "We found two penises in the fossil," reported Professor Siveter.

So why do we need to know when penises arose on the evolutionary calendar?

"I think it's a bit of an academic exercise," said Mr Robinson. "In the animal world, whatever works is good enough - you don't need to improve on it unless it improves the chances of fertilisation."

That said, penises come in all shapes and sizes, and the differences between vertebrate penises and invertebrate penises are quite significant.

"They perform the same function but they are derived from different tissue, different appendages", said Mr Robinson.

For example, the invertebrate mating system - nicknamed the 'lock and key design' - is a useful tool for species identification.

"The male phallus has spines and a knob and the female aperture has a series of baffles and notches. It's really like a key in a lock - only the one key will fit inside the lock," he said.

"As a result, cross-breeding between similar species that often inhabit the same ecological niche is inhibited."